1. Field of the Invention
The present invention relates generally to joints between aluminum and steel bodies and more particularly, to a strong joint capable of withstanding large tensile forces while minimizing corrosion problems.
2. Description of the Prior Art
Steel and aluminum materials frequently need to be joined in applications where the particular characteristics of each of the two metals are desirable for various reasons. For example, steel is known to be a stronger material than aluminum, while aluminum is lighter than steel. So in instances where strength and weight are critical parameters, aluminum may be employed to keep the weight down, while steel is employed to give added strength. It is known, however, that steel cannot be fusion-welded to aluminum and accordingly, other methods of joining the two materials have been developed. A mechanical connection sometimes provides the desired strength, but presents a problem of oxidation and/or galvanic corrosion due to the exposure of the crevice at the aluminum/steel joint to environmental elements. It has been demonstrated that an explosion-welded joint, by eliminating the crevice between the steel and aluminum, reduces the rate of corrosion to acceptable levels. The explosion-welding technique cannot always be employed at all locations, thereby necessitating a modification of the steel and aluminum components to facilitate an explosion-welding process. Explosion-welding is a known technique and is fully described in U.S. Pat. No. 3,137,937 issued to G. R. Cowan, et al. on June 23, 1964, and U.S. Pat. No. 3,364,561 issued to J. Barrington on Jan. 23, 1968.
An illustration of an instance wherein aluminum and steel are joined for practical reasons, is in modern-ship design which makes extensive use of aluminum structural components for weight reduction, while utilizing steel for structural components that require greater strength or toughness. As mentioned previously, the joining of aluminum and steel components presents a design problem in that the two metals cannot be directly fusion-welded to one another. Any mechanical joint creates a problem with galvanic corrosion in the crevice between the two components. A typical situation where the problem occurs is in the joining of aluminum super structures to steel decks. This design has been employed extensively by the United States Navy in warships.
For approximately the last 15 years, the problem of galvanic corrosion in aluminum/steel joints has been avoided by the use of explosion-welded tri-metallic transition joints. This system employs a plate or bar composed of a layer of steel, a layer of pure aluminum, and a layer of an aluminum alloy being used in the ship structure. The layer of aluminum is required to improve the strength of the joint as explosion welds between steel and aluminum alloys do not result in a very strong joint.
The steel/aluminum/aluminum alloy joint solves the corrosion problem and has proven satisfactory in situations where components are to be held together with only moderate separating forces, such as pulling forces, being applied and where the joined components do not need to be disassembled for maintenance or other purposes. However, in situations where concentrated separating forces are applied, the steel/aluminum/aluminum alloy joint is not satisfactory because the joint is welded to the components. An example of such a situation is the need to attach helicopter recovery systems to aluminum decks using high strength steel bolts. Rough sea recovery operations exert very high pulling forces on the recovery system and accordingly, a very strong joint to the deck is required. Additionally, the recovery system requires periodic disassembly for system maintenance.
A primary objective of the present invention is, therefore, to provide a strong transition joint between an aluminum plate and a steel object such as a bolt.
A second objective is to make a joint that provides easy disassembly of the joined components.
A third objective is to make the joint highly resistant to galvanic and atmospheric corrosion.